PRIMATE SOCIAL SYSTEMS CROOM HELM 'Studies in Behavioural Adaptation' Edited by John Lazarus, University of Newcastle upon Tyne Gulls and Plovers: The Ecology and Behaviour of Mixed­ Species Feeding Groups C.J. Barnard and D.B.A. Thompson (Croom Helm, London & Sydney/Columbia University Press, New York) Modelling in Behavioural Ecology: An Introductory Text Dennis Lendrem (Croom Helm, London & Sydney/Dioscorides Press, Portland, Oregon) Social Behaviour in Fluctuating Populations Andrew Cockburn (Croom Helm, London, New York, Sydney) PRIMATE SOCIAL SYSTE_ ROBIN I,M. DUNBAR Department of Zoology University of Liverpool CROOM HELM London & Sydney ~ 1988 Robin Dunbar Softcover reprint of the hardcover 18t edition 1988 Croom Helm Ltd, Provident House, Burrell Row, Beckenham, Kent BR3 1AT Croom Helm Australia, 44-50 Waterloo Road, North Ryde, 2113, New South Wales British Library Cataloguing in Publication Data Dunbar, R.I.M. Primate social systems. 1. Primates-Behavior 2. Mammals- Behavior 3. Social behavior in animals I. Title 599.8'04524 QL737.P9 ISBN -13: 978-1-4684-6696-6 e-ISBN-13: 978-1-4684-6694-2 DOl: 10.1007/978-1-4684-6694-2 Typeset in lOpt Plantin by Leaper & Gard Ltd, Bristol, England Billing & Sons Limited, Worcester. Contents Series Editor's Foreword Preface 1. Primates and their Societies 1 Asking the Right Questions 3 The Primate Heritage 5 Primate Social Systems 9 2. Theory of Reproductive Strategies 15 An Evolutionary Perspective 15 Optimal Strategy Sets 21 A Question of Ontogeny 24· Structure and Function in Primate Society 28 3. Survival Strategies 33 Nutritional Requirements 33 Dietary Strategies 38 Optimal Foraging 41 Exploiting the Habitat 44 Economics of Territoriality 50 The Problem of Predation 53 4. Demographic Processes: (1) Lifehistory Variables 55 Lifehistory Variables 56 Variance in Birth Rates 63 Mortality Rates 70 5. Demographic Processes: (2) Population Parameters 76 Population Dynamics 76 Migration and Fission 80 Sex Ratio 85 6. Time Budgets and Other Constraints 90 Time Budgets 90 Demographic Constraints on Behaviour 96 7. Evolution of Grouping Patterns 106 Why Form Groups? 106 Costs of Group-living 113 Evolution of Groups 116 Evolution of Social Structure 137 8. Mating Strategies 151 Reproductive Behaviour in Primates 151 Gaining Access to Mates 156 Male Lifetime Reproductive Success 167 Alternative Strategies of Mate Acquisition 172 v vi CONTENTS 9. Rearing Strategies 182 Primate Rearing Patterns 182 Ecology of Motherhood 189 Social Aspects of Rearing 198 Male Parental Investment 202 10. Conflicts and Coalitions 205 The Constraint-free Strategy 205 Economics of Coalition Formation 214 Demographic Considerations 221 Acquisition of Rank 226 11. Mechanics of Exploitation 237 Processes of Negotiation 237 Dynamics of Social Relationships 243 Role of Communication 248 Exploiting Others 254 Infanticide as a Reproductive Strategy 257 12. Socio-ecological Systems 262 Models as Descriptive Tools 262 Models as Analytical Tools 267 The Problem of Monogamy 273 Comment:The Function of Territoriality 288 A Note on the Use of Modelling 290 13. Evolution of Social Systems 292 Social Evolution in Baboons 293 Social Evolution in the Great Apes 305 Conclusion 323 Appendix 326 Scientific and Common Names 327 References 330 Index 371 Preface This book grew from small beginnings as I began to find unexpected patterns emerging from the data in the literature. The more I thought about the way in which primate social systems worked, the more interesting things turned out to be. I am conscious that, at times, this has introduced a certain amount of complexity into the text. I make no apologies for that: what we are dealing with is a complex subject, the product of evolutionary forces interacting with very sophisticated minds. None the less, I have done my best to explain every­ thing as clearly as I can in order to make the book accessible to as wide an audience as possible. I have laid a heavy emphasis in this book on the use of simple graphical and mathematical models. Their sophistication, however, is not great and does not assume more than a knowledge of elementary probability theory. Since their role will inevitably be misunderstood, I take this opportunity to stress that their function is essentially heuristic rather than explanatory: they are designed to focus our attention on the key issues so as to point out the directions for further research. A model is only as good as the questions it prompts us to ask. For those whose natural inclination is to dismiss modelling out of hand, I can only point to the precision that their use can offer us in terms of hypothesis-testing. Inevitably with a book like this, its final form owes a great deal to many different people, most of whom contributed to it inadvertently. The manu­ script was produced while I was supported by a University Research Fellow­ ship from the University of Liverpool, and I am grateful to the Department of Zoology for providing me with the resources necessary for undertaking a pro­ ject like this. The text itself benefited from discussions with both students and colleagues, in particular Geoff Parker. I am especially indebted to John Lazarus and Sandy Harcourt for their detailed comments on the manuscript, as well as to Robert Hinde and Saroj Datta for their comments on specific chapters. Many other individuals generously provided me with access to unpublished data or drew my attention to papers that I would otherwise not have seen: in particular, I thank Jeanne Altmann, Dick Byrne, Caroline Harcourt, Julie Johnson, Bill McGrew, Thelma Rowell, Robert Seyfarth, Joan Silk and Andy Whiten. I am also very grateful to those who provided me with unpublished data on their baboon study populations, in particular Connie Anderson, Tony Collins, Juliet Oliver, ·Dennis Rasmussen, Hans Sigg, Alex Stolba and, of course, Martin Sharman who worked with me on the analysis of these data. Finally, this book owes a great deal to the influence of John Crook, Hans Kummer and Thelma Rowell: in many ways, it stands as a tribute to their influence on the field. Robin I.M. Dunbar vii Series Editor's Foreword In the early years of this century a Scottish doctor speculated on the evolutionary origin of human tears. It seemed to him that with the increase in brain size and cognitive powers of our early ancestors many events in the struggle for existence would be just too distressing to witness. How comforting then for a mother, distraught by the sight of her child being devoured by a lion, to cloud her vision with a flood of tears! Just so, though if the good doctor had pondered further, the following picture might have occurred to him, comfortable in his speculative armchair, and given him some pause for thought. These stories do not, of course, get us very far in explaining the evolution of tears - or any­ thing else - but they do remind us how far the study of behavioural adaptation has come this century. This is, in fact, an exciting time for students of behaviour. The last twenty years has seen a great advance in the theoretical armoury for tackling problems of behavioural evolution and adaptation, and a parallel expansion in empirical studies, particularly in the field. The concepts of inclusive fitness and evolutionary stability, for example, have helped to explain major features of social behaviour and have generated entirely new questions for the field worker to examine. Cost­ benefit analysis and optimisation theory have done the same for behaviour in general, and links with population biology and population genetics are becoming stronger. The heady days which saw the birth of behavioural ecology and sociobiology are now over, the new concepts have been refined and consolidated, and field data and comparative studies have accumulated at an impressive rate. Now seems a good time to take stock, to review the state of the art and to point some directions for future work. These are the aims of the present series, which will examine questions of behavioural adaptation and evolution in depth. As for our intended readership, we hope that all those interested in such problems, from advanced under­ graduate to research worker and lecturer, will find these books of value. Some contributions to the series will encompass particular areas of study, reviewing theory and data and presenting fresh ideas. Others will report the findings of new empirical studies of an extensive nature, which make a significant contribution by examining a range of interrelated questions. The richness, but also the difficulty, of functional enquiry results from the multiple effects of selection pressures and the complex causal relationships between the behavioural responses to evolutionary forces. Studies which measure a comprehensive set of behavioural attributes, and their ecological corre­ lates, are therefore particularly valuable. Understanding the evolution of whole social systems is the most ambitious task of all for behavioural ecology, and the social systems of the primates present a particular challenge, due to their complexity and the subtlety of the behavioural tactics that underpin them. Taking up this challenge here, Robin Dunbar develops his account of the evolution of primate societies by testing data from the primate literature with general theories of grouping, mating, rearing and so on, taking into account the constraints of ecology and evolutionary history imposed on the group. In this way the literature is presented not as piecemeal information to be digested, and perhaps understood post hoc, but as material for the corroboration or dismissal of hypotheses about social life. Robin Dunbar's account is therefore an ideal student introduction to the evolutionary complexities of primate social structure, and to the methods of hypothesis testing in behavioural ecology. However, it is also much more than this. A particularly important feature of the book is the demonstration of the profound influence that simple demographic variables such as birth rate can have on social structure, and in this area it breaks new ground.